Thomas Menino is a powerful machine politician from a city that’s been churning them out for over a century. In five terms as mayor of Boston, Menino has used alliances cultivated over decades to reward allies, punish enemies and squelch dissent. This old-school style has served both him and the city well, as evidenced by his nearly 80% approval rating. Yet as voters head to the polls today to pick a new mayor for the first time in a generation, the city he has led for 20 years appears set to make a sharp break.

Twelve candidates are jockeying to succeed Menino in today’s preliminary election (the top two will face off on Nov. 5), and none are looking to become a boss in his mold. Boston’s political structure has tended to reward entrenched connections and political dynasties. Menino rose through the ranks as a driver for a state senator, and he became mayor in part because he cultivated a close relationship with his predecessor while serving on the city council. But in the race to succeed Menino, the politicians most indebted to his machine have scuffled, while those outside of it have surged.

This is partly because the job of a big-city mayor is far different than it was just two decades ago, when Menino took office. Urban mayors in the ’70s, ’80s and early ’90s were battling crime, disinvestment and suburban flight that had hollowed out their cities. Now people and money are flowing into cities, and the focus of their leaders is less on stemming decline than on managing and harnessing growth.

Menino has worked to keep pace with the changing times. He recognized the rise of the technology sector and recruited tech companies to relocate to Boston’s waterfront. His staff is developing mobile constituent service apps, and he has made a mini-cause of parklets — tiny pockets of plants and seating temporarily installed over street-parking spaces. And he recently dropped a 20-year prohibition on voicemail at city hall — a particularly tough concession for a natural glad-hander who is said to have met nearly half of the city’s residents. “The city’s changing all the time,” Menino says. “And as mayor, you have to try to change with it.”

It’s a far different city from the one Menino inherited when he took office in 1993. Boston then was just beginning to stanch decades of heavy population loss and was in the grip of a ruinous murder wave. It has become younger and livelier in the decades since, with the center of gravity shifting from the ethnic neighborhoods toward the downtown and waterfront. To meet the demand, long-vacant industrial parcels are springing to life as apartment towers and offices. And the pace of growth is quickening: Boston added more residents from 2010 to 2012 than it did during the entire 1990s.

Menino has always been a self-styled urban mechanic, consumed by the nuts and bolts of local government. He traveled the city’s neighborhoods, dropping by local businesses, shaking residents’ hands and flagging potholes and graffiti on his way to the office. But as the job of a big-city mayor has evolved, Boston voters seem ready to move beyond the Menino era. In 2013, there are far more efficient ways to get a pothole filled than to have the mayor see it on the way to work.

Tellingly, most of the dozen candidates vying to succeed Menino are proposing more than a break in style. Mike Ross has proposed partnering with neighboring Cambridge and Somerville to pitch the Boston region to national and international companies — a contrast to the Menino administration’s use of tax breaks to lure companies across the Charles River, from Cambridge to Boston. Another candidate, John Connolly, talks about making city government work like an Apple store — online and painless.

Connolly, a young, Harvard-educated city-council member, is a heavy favorite to advance to the November election. He has appealed to middle-class voters and urban transplants with an emphasis on education reform, building more middle-class housing and laying down an extensive bicycle-lane network. Much of his platform is devoted to ideas for sustaining the growth fostered under Menino.

“How do you capitalize on the urbanization trend?” Connolly says. “Young people are coming here. How do we get them to stay for a lifetime, and build the city’s future?”

Marty Walsh shows how drastically Boston’s politics have shifted in recent years. Walsh is a second-generation laborer, state legislator and former union official. He’s the son of Irish immigrants and a native of Dorchester, an ethnically and economically diverse neighborhood. And even though Walsh looks like the oldest of Old Boston politicians, he’s impossible to caricature. Walsh cruises art galleries and marvels at Kansas City’s speedy Internet service. His campaign account is full of labor money, but he has a frosty relationship with the powerful teachers’ union, and he’s as strong among the progressive Boston activists who launched Elizabeth Warren into the U.S. Senate as he is with the hard hats with whom he grew up.

Another candidate who defies the traditional political order is John Barros, a son of Cape Verdean immigrants who rose from one of Boston’s poorest neighborhood to college at Dartmouth and a job underwriting IPOs on Wall Street. For the past decade, Barros has headed a nonprofit that’s part affordable-housing developer, part community organizer. The first-time candidate’s campaign has combined antipoverty calls with a prodevelopment bent more reminiscent of traditional downtown candidates. In six months, Barros went from political unknown to receiving the endorsement of the Boston Globe.

Candidates with the closest ties to Menino, meanwhile, have struggled. Rob Consalvo, a city-council member often described as Menino’s surrogate son, and Charlotte Golar Richie, the mayor’s former housing chief, have failed to generate much excitement among voters despite being closer to the popular mayor than anyone else in the race. These things used to matter a great deal in Boston politics. Not anymore.